I have never blamed Diana for not being at the graveside. I think her attending the service was more then enough.
Her actions inside the church though were extremely ill advised. She probably called upon Mary for moral support, problem being Mary had not been informed and was rightly outraged. It is possible it was a spur of the moment decision, but still makes for appalling etiquette. Shades of Motown 25.
I wonder what would have happened had it been Mary who had asked Diana to join her in a private prayer. Would the media have reacted the way they did?. Would Diana have played the diva and refused?.